In recent years, along with the development of portable electronic equipment such as a mobile phone and a notebook personal computer, and the commercialization of electric automobiles, there is a demand for further enhancement of performance and stability of a secondary battery and a capacitor to be used as power sources for the above-mentioned equipment and electric automobiles. In particular, a lithium secondary battery is drawing attention as a battery having high energy density and is being improved from various viewpoints as a preferred power source for the above-mentioned equipment and electric automobiles.
In the above-mentioned lithium secondary battery, various improvements are still being made for the purpose of enhancing battery characteristics. For example, Patent Document 1 discloses a lithium secondary battery in which the gas generation amount is reduced and the storage characteristics at high temperature are rendered excellent by causing a nonaqueous electrolytic solution to contain a particular compound based on a phosphate ester.
Further, Patent Document 2 discloses a lithium secondary battery in which the swelling characteristics at high temperature are ameliorated by adding a phosphonate compound containing an unsaturated hydrocarbon radical to an electrolytic solution.
Further, lithium cobaltate (LiCoO2) is easy to produce and handle, and hence is used frequently as a preferred positive active material for a lithium secondary battery. However, LiCoO2 is produced using cobalt (Co) that is a rare metal as a raw material, and therefore, it is expected that resource shortage will become a serious problem. Further, the price of cobalt is high, and the price fluctuation thereof is large. Therefore, it is desired to develop a positive electrode material that can be supplied stably at low cost.
Under the above-mentioned circumstance, a positive active material replacing LiCoO2 is being developed, and for example, Patent Document 3 discloses a positive active material containing nickel (Ni), manganese (Mn), cobalt (Co), and another substitution element M, in which a containing ratio of each element is defined, and an atomic ratio “a” of the substituent element M with respect to Mn, Ni, and Co on the surface of a positive active material particle is set to be larger than an average atomic ratio of the substituent element M with respect to Mn, Ni, and Co in the entire positive active material particle. The above-mentioned positive active material has a capacity higher than that of LiCoO2, and in particular, the positive active material containing Ni has a high capacity in the vicinity of 3 to 4.2 V on the basis of a lithium metal. Thus, the positive active material containing Ni is a material capable of further increasing a battery capacity.